Method and device for the homogeneous heating of glass and/or glass-ceramic articles using infrared radiation.
The invention relates to a process for the homogeneous heating of semi-transparent and/or transparent glass articles and/or of glass-ceramic articles with the aid of infrared radiation, whereby the glass articles and/or the glass-ceramic articles undergo a heat treatment in the range from 20° C. to 3000° C., as well as to a device for the homogeneous heating of translucent and/or transparent glass articles and/or glass-ceramic.
Semi-transparent or transparent glass and/or glass-ceramics, for the setting-in of certain material properties, for example ceramization, are heated mostly to temperatures which lie preferably over the lower cooling point (viscosity Z=1014.5 dPas). In form-giving processes, especially hot after-processing, the semi-transparent or transparent glass and/or the glass-ceramic material is heated up to the processing point (viscosity Z=104 dPas) or beyond that. Typical lower cooling points can amount, depending on the type of glass, to between 282° C. and 790° C., and typically the processing point can be up to 1705° C.
Hitherto according to the state of the art semi-transparent or transparent glasses and/or glass-ceramics, for example for ceramization, were heated preferably with surface heating. As surface heating there are designated processes in which at least 50% of the total heat output of the heat source is introduced into the surface or surface-near layers of the object to be heated.
If the radiation source is black or gray and if it has a color temperature of 1500 K, then the source radiates off 51% of the total radiation output in a wavelength range above 2.7 μm. If the color temperature is less than 1500 K, as in most electric resistance heating elements, then substantially more than 51% of the radiation output is given off above 2.7 μm.
Since most glasses in this wavelength range have an absorption edge, 50% or more of the radiation output is absorbed by the surface or in surface-near layers. It is possible, therefore, to speak of surface heating. Another possibility lies in heating glass and glass-ceramics with a gas flame, in which typical flame temperatures lie at 1000° C. Such a heating occurs mainly by direct transfer of the thermal energy of the hot gas onto the surface of the glass or of the glass-ceramic, so that here it is possible to proceed from a predominantly surface/superficial/heating.
In general with the earlier described surface heating the surface or surface-near layers are heated in the parts of the glass or of the glass-ceramic that lie opposite the heating source. The remaining glass volume or glass-ceramic volume must accordingly be heated up correspondingly by heat conduction within the glass or the glass-ceramic material.
Since glass or glass-ceramic material has as a rule a very low heat conductivity in the range of 1 W (m K), glass or glass-ceramic material must be heated up more and more slowly in order to keep tensions in the glass or glass-ceramics low.
A further disadvantage of known systems is that, in order to achieve a homogeneous heating-up of the surface, the surface of the glass or of the glass-ceramic material must be covered as completely as possible with heating elements. Limits are placed there on conventional heating processes. With electrical heating resistances made of Kanthal wire, as they are preferably used, at 1000° C., for example, only a wall load of maximally 60 kW/m2 is possible, while a full-surfaced (or holohedral) black radiator of the same temperature could irradiate an output density of 149 kW/m2.
With a denser packing of the heating elements to be equated with a higher wall load, these would heat themselves up reciprocally, which through the resulting heat accumulation would involve an extreme shortening of the useful life of the heating elements.
When a homogeneous heating-up of the glass or of the glass-ceramic is not achieved or is only inadequately successful, then this unfailingly results in inhomogeneities in the process and/or in the product quality. For example, any irregularity in the process conducting, in the ceramization process of glass-ceramics leads to a cambering or bursting of the glass-ceramic article.
From DE 42 02 944 C2 there has become known a process and a device comprising IR radiators for the rapid heating of materials which have a high absorption above 2500 nm. In order to rapidly introduce, into the material, the heat given off from the IR radiators, DE 42 02 944 C2 proposes the use of a radiation converter from which secondary radiation is emitted with a wavelength range which is shifted into the long-wave direction with respect to the primary radiation.
A heating of transparent glass homogeneous in depth with use of short-wave IR radiators is described in U.S. Pat. No. 3,620,706. The process according to U.S. Pat. No. 3,620,706 is based on the principle that the absorption length of the radiation used in glass is very much greater than the dimensions of the glass object to be heated, so that the major part of the impinging radiation is let through by the glass and the absorbed energy per volume is nearly equal at every point of the glass body. What is disadvantageous in this process, however, is that no homogeneous irradiation over the surface of the glass objects is ensured, so that the intensity distribution of the IR radiation source is depicted on the glass to be heated. Moreover, in this process only a small part of the electric energy used is utilized for the heating of the glass.
The problem of the invention, therefore, is to give a process and a device for the homogeneous heating-up of semi-transparent or transparent glass and glass-ceramic articles, with which the aforementioned disadvantages are overcome.